Protesters hold signs during a march in support of the girls kidnapped by members of Boko Haram in front of the Nigerian Embassy in Washington May 6, 2014.

Nigerian police offered a $300,000 reward on Wednesday for information leading to the rescue of more than 200 schoolgirls abducted by Islamist rebels.

The mass kidnapping last month by militant group Boko Haram, which is fighting for an Islamic state in northeast Nigeria, triggered an international outcry and protests in Nigeria, piling pressure on the government to get the girls back.

Public anger mounted after locals on Tuesday said another eight girls had been seized from the same remote northeastern area by suspected members of the group.

Police listed six phone numbers in their statement and urged Nigerians to call in with "credible information".

Boko Haram leader Abubakar Shekau has threatened in a video to sell the girls abducted on April 14 from a secondary school in the village of Chibok "on the market".

The United States has offered to send a team to Nigeria to help the search efforts.

Suspected Boko Haram gunmen kidnapped eight more girls from a village near one of the Islamists' strongholds in northeastern Nigeria on Monday night.

The kidnappings, and other attacks by Boko Haram, have overshadowed Nigeria's hosting of the World Economic Forum, which is due to start on Wednesday evening. Nigerian officials had hoped the event would draw attention to the potential of Africa's biggest economy as an investment destination.

Last month's kidnapping occurred on the day a bomb blast, also claimed by Boko Haram, killed 75 people on the outskirts of Abuja, the first attack on the capital in two years. Another bomb in roughly the same place killed 19 people last week.

Nigerian President Goodluck Jonathan welcomed the U.S. offer to send an American team to Nigeria to support the government's efforts to find the girls.

Obama told ABC the kidnappings "may be the event that helps to mobilize the entire international community to finally do something against this horrendous organization that's perpetrated such a terrible crime."

Britain has also offered to help, but Foreign Minister William Hague told Sky News on Wednesday that would be "difficult because this is primarily a matter for Nigeria".

Kassim Shettima, governor of Borno state, where the insurgency is most active, was due to visit the scene of a deadly attack on the market town of Gamburu on Wednesday.

At least 13 people died in that attack on Monday morning by suspected Islamist rebels, who also torched vehicles and houses.